understructure vs fundament

understructure and fundament both are nouns.

Word	Noun	Adjective	Verb	Adverb
understructure	Yes	No	No	No
fundament	Yes	No	No	No

As nouns, understructure and fundament are synonyms defined as:

Other synonyms of understructure include base, foot, foundation, groundwork, substructure.

understructure (noun)	fundament (noun)
lowest support of a structure	lowest support of a structure
	the fleshy part of the human body that you sit on
	the fundamental assumptions from which something is begun or developed or calculated or explained